The rotary output shaft or rotor of conventional downhole motors used in the oil drilling industry moves in an eccentric manner. In order for the drill bit to perform properly, this eccentric motion must be converted into a concentric motion. This conversion is typically accomplished by using a drive shaft with universal joint connections between the downhole motor and the bearing assembly that rotates the drill bit. Progressive cavity well pumps also have rotors that move in an eccentric manner.
In order to extend the useful life of the universal joints, seals are used to prevent abrasive drilling fluids from entering the joints. Early seals were relatively simple, comprising a flexible sleeve, typically elastomeric, placed over the entire universal joint assembly. Once the sleeve was installed, the joint assembly was filled with lubricating fluid under appropriate pressure. Although this sealing method was inexpensive and easily installed, the sleeves were prone to slide out of position or to become caught between the joint members, or to burst due to pressure fluctuations. As a result, drilling fluid would penetrate the joint, causing the premature failure of the joint.
U.S. Pat. No. 5,267,905 discloses a universal joint assembly with polypack sealing elements to prevent drilling fluid from entering the joint. Springs installed in the retainer of the joint push the seals against the joint housing regardless of the angular displacement of the shaft within the housing. Drilling fluid communicates with the lubricant to equalize the pressure within the lubricant area with the hydrostatic pressure of the drilling fluid.
U.S. Pat. No. 5,704,838 discloses a universal joint assembly with a multi-component seal to keep drilling fluid out of the joint. The seal structure comprises a seal element and a seal carrier arrangement. The seal carrier arrangement includes a carrier member, biasing means such as a Belleville spring, and a retaining ring to hold the carrier member in place. Two O-ring seals positioned on the carrier arrangement work with the seal element to prevent drilling fluid from entering the joint.
While these seal arrangements are superior to the flexible sleeve, further improvements are desired. In some instances, the joints have been unsuitable for use at high operating temperatures because of the types of seal elements used. Also, rotational angular misalignment between the shaft and the housing may cause some types of seals to exhibit permanent deformation and to fail if worked beyond the fatigue limit.